1. Field of the Invention
The present invention relates generally to a method of inducing flat frequency modulation response characteristics in a semiconductor laser, and also to an electrode arrangement for a semiconductor laser for attaining such a flat modulation frequency response.
2. Description of the Prior Art
In optical fiber communication systems, a transmission rate up to 2.5 Gb/s (Giga bits per second) is currently possible and the trend toward even higher rates is accelerating.
In particular, optical heterodyne transmission technologies, which are now under development, utilize a semiconductor laser as a single-mode light source. Such an optical heterodyne transmission allows signal receiver sensitivity to increase in the order of one digit as compared with direct detection techniques and, further is able to render optical frequency division multiplexing a practicality. This latter mentioned technique is seen as the key to the next generation of optical or coherent communications systems.
In such an optical heterodyne communications system, three techniques are utilized for light modulation: FSK (Frequency Shift Keying), PSK (Phase Shift Keying) and ASK (Amplitude Shift Keying). As is known, modulation of a current, injected into a semiconductor laser, changes a refractive index of the laser, which results in the light output frequency being modulated by the current injected into the semiconductor laser.
According to FSK, an optical signal to be detected by heterodyne detection techniques, can be produced merely by modulating an injection current to a semiconductor laser. FSK features that (a) a transmitter can be configured in the same manner as in ASK and (b) receiver sensitivity is improved as compared with ASK. Accordingly, the research and development of FSK modulation of a semiconductor laser are extensive.
In order to practically realize such a direct frequency modulation (FM) using a semiconductor laser, it is necessary to fabricate a laser whose optical output signal has a narrow spectral linewidth and whose modulation frequency response is flat over a wide range.
The instant invention is directly concerned with an improvement for achieving a wide range of flat frequency response in a direct frequency modulation of a semiconductor laser.
As is known, if the frequency of a modulation current is less than several hundreds of KHz, the oscillation frequency of a semiconductor laser depends on a thermal effect wherein a refractive index of the laser increases with increase in the modulation current. This frequency deviation is referred to as a red-shift, viz., a shift to a lower frequency with increase in the modulation current. On the other hand, in the case where the frequency of the modulation current is more than several MHz, a carrier effect becomes dominant and the oscillation frequency of the semiconductor laser shifts to a higher frequency with increase in the modulation current. This is referred to as a blue-shift.
However, in order to achieve optical heterodyne communication, even if the frequency of a modulation current varies from a few KHz to several hundreds MHz, a frequency modulation response should be flat over a wide modulation frequency range.
The above-mentioned problems inherent in a semiconductor laser, have been discussed in detail in a Japanese paper entitled "Modulation and Demodulation Technology for Coherent Optical Fiber Transmission--FSK Heterodyne Detection" by Yoshihisa YAMAMOTO and Tatsuya KIMURA, a practicing report of researches, volume 31, No. 12, pages 2173-2184 issued by Nippon Telegraph & Telephone (NTT) Corporation (prior art paper No. 1).
According to a known approach to solving the aforesaid problems, an electrode of a Distributed Feedback (DFB) type semiconductor laser is divided into a plurality of sections. The bias conditions of the currents applied to the plural electrodes are appropriately controlled. This approach has been proposed in a Japanese paper entitled "Bias Dependence of FM Response in Three-electrode Long Cavity DFB-LD" by H. Miyata, et al, No. C-143 of 1990 Spring General Assembly of Japanese Electronic Information Communication (prior art paper No. 2). However, this prior art has encountered the difficulty that the bias conditions are strictly limited for exhibiting a satisfied frequency response.
Other known efforts to overcome these problems are disclosed in a paper entitled "Spectral Characteristics for a 1.5 .mu.m DBR Laser with Frequency-Tuning Region" by S. Murata, et al., IEEE Journal of Quantum Electronics, Vol. QE-23, No. 6, June 1987, pages 835-838 (prior art paper No. 3). According to paper No. 3, the laser structure of a Distributed Bragg Reflector (DBR) type laser is divided into three regions (viz., action region, phase control region, DBR region) which are independently provided with electrodes. The FM is implemented within the phase control and DBR regions each of which has no light emitting layer. However, according to this prior art, the frequency response is undesirably restricted to a few hundred MHz due to carrier lifetime.
Still further, Japanese Patent Application No. 63-50205, which was primarily published Sep. 6, 1989 under publication No. 1-223791, discloses a four-electrode DFB type semiconductor laser (prior art paper No. 4). According to this publication, a phase control region is provided with two electrodes one of which is reverse biased, and a frequency modulation is implemented in the reverse biased region. However, this prior art disclosed in Japanese Patent Application No. 63-50205 has encountered the difficulty that a suitable efficiency of frequency modulation is not expected in that the refractive index variation in the reversed bias region is insufficient.
Before turning to the instant invention, it is deemed preferable to discuss a known semiconductor structure to which a method of the instant invention is applicable.
FIG. 1A is a perspective view of a known multi-electrode DBR type semiconductor laser chip 10, while FIG. 1B is a cross sectional schematic of the laser chip 10 of FIG. 1A viewed at section I-I'.
As best shown in FIG. 1B, the laser chip 10 includes three regions: an active region 12 (300 .mu.m long), a phase control region 14 (150 .mu.m long), and a DBR region (500 .mu.m long). An Sn-doped n-InP substrate 18 carries a grating 20 (240 nm pitch, 7.times.10.sup.17 /cm.sup.3 carrier density) on the top surface thereof at the DBR region 16. A non-doped InGaAsP active layer 22 (0.1 .mu.m thickness, 1.55 .mu.m wavelength) is formed on the substrate 18 in the active region 12, while an InGaAsP wave guide layer 24 (0.2 .mu.m thickness, 1.3 .mu.m wavelength) is present on the regions 14, 16. A p-InP cladding layer 26 (2 .mu.m thickness, Zn-doped, 1.times.10.sub.18 /cm.sub.3 carrier density) is formed on the active layer 22 and the wave guide layer 24.
Although not illustrated in FIG. 1B, a Zn-doped p-InGaAsP contact layer (0.5 .mu.m thickness, 1.times.10.sup.18 /cm.sup.3 carrier density) is grown over the entire region of the cladding layer 26, after which an electrode layer of Cr/Au is formed on the entire surface of the contact layer. The contact and electrode layers undergo preferential etching to be divided into three electrically isolated electrodes. As shown in FIG. 1B, the divided contact layers of the regions 12, 14 and 16 are respectively denoted by 28a, 28b and 28c, while the divided electrodes of the regions 12, 14 and 16 are represented by 30a, 30b and 30c, respectively.
Further, another electrode 32 is formed on the side of the substrate 18, which is opposite to the active layer 22 and the wave guide layer 24.
For the purpose of transverse mode control, a mesa stripe 34 is formed, after which Fe-doped, InP layers 36 and 38 are formed on either side thereof (see FIG. 1A).
The laser chip 10 is mounted on a diamond heat sink (not shown) with the p-type side up configuration. An oscillation threshold current, a differential quantum efficiency, a maximum light output are, 20 mA, 0.15 W/A and 30 mW, respectively. The active region 12 is injected so as to obtain a light output of 10 mW, wherein an oscillation wavelength is 1.550 .mu.m.
Reference is made to FIG. 2, wherein a method of directly modulating frequency of a light output is schematically illustrated. A laser chip structure shown in FIG. 2 is identical with that of FIGS. 1A and 1B. Accordingly, like numerals are used and redundant descriptions will be omitted for brevity.
As mentioned above, if the frequency of a modulation current is less than several hundreds KHz, the oscillation frequency of a semiconductor laser encounters a thermal effect wherein a refractive index of the laser increases with increase in the modulation current (viz., red-shift). On the other hand, in the event that the frequency of the modulation current exceeds several MHz, the oscillation frequency of the semiconductor laser shifts to a higher frequency with increase in the modulation current due to carrier effect (viz., blue-shift).
Thus, if a modulation current is injected into the active region 12 as shown in FIG. 2, the frequency modulation response may be plotted as indicated by a chained line 50 in FIG. 4. It is clearly understood that a flat frequency modulation response cannot be expected over a wide range.
On the other hand, if a modulation current is injected into the phase control region 14 as shown in FIG. 3, the frequency modulation response of the nature depicted by the broken line trace 52 in FIG. 4. In this instance, the frequency deviation is approximately 2 GHz/mA which is a few times greater than the case depicted in FIG. 3 (viz., the case wherein the modulation current is injection into the active region 12). However, the frequency modulation response is limited by carrier lifetime and thus the upper limit thereof is only about 600 MHz.